Method for reclaiming polyester or polyester mixtures

ABSTRACT

The invention relates to a method for reclaiming polyesters or polyester mixtures from polyester or waste containing polyester. The aim of the invention is to reclaim polyesters or polyester mixtures of good quality in a cost-effective manner. To achieve this aim, the polyester and/or waste containing polyester is heated in a bath of dicarboxylic acid dialkyl ester to temperatures in excess of 150° C. In this process, polyester is dissolved in the dicarboxylic acid dialkyl ester or a dicarboxylic acid dialkyl ester mixture and subsequently extracted from the solution by means of solid-liquid separation.

The invention relates to a method of reclaiming polyesters or polyestermixtures from polyester or waste containing polyester. Polyesters haveproved to be particularly suitable plastics materials for manyapplications. Thus they are used in the food industry as packaging orfor containers. Another important field of use for polyester plastics isin printed circuit boards for the electronics industry.

Since high quality requirements are set for the recycling of plasticsfrom polyester limits have been set on recycling by the achievablequality or the financial outlay for reprocessing. Cost and quantity areaffected by waste guided in the recycling circuit, which is partiallyprinted, has labels glued to it or is contained in the form of compositematerials, as in the case of printed circuit boards.

The object of the invention, therefore, it to propose a possible way ofreclaiming polyesters or polyester mixtures of a good quality in acost-effective manner, thus saving natural resources.

This object is accomplished according to the invention with the featuresof claim 1. Advantageous embodiments and developments of the inventionarise with the features mentioned in the subordinate claims.

In the method according to the invention for reclaiming polyesters orpolyester mixtures from polyester or waste containing polyester, theprocedure is that the corresponding pre-sorted waste is heated up in abath of dicarboxylic acid dialkyl ester, the polyester portions beingdissolved at a temperature in excess of 150° C., preferably in excess of180° C., and by very particular preference just below the boiling pointof dicarboxylic acid dialkyl ester or a dicarboxylic acid dialkyl estermixture, said boiling point lying at approx. 210°.

In this process, the waste should be previously cleaned and rid ofimpurities, it being possible to carry out sieving and/or washingpossibly with heavy-liquid separation.

It is also advantageous to comminute the waste mechanically before thedissolving process, such that the surface conditions for dissolving areimproved.

A particularly suitable dicarboxylic acid dialkyl ester mixture hasproved to be the one which is commercially available from the companyDuPont under the designation DBE.

The dissolved polyesters can be separated from the dicarboxylic aciddialkyl ester solution by mechanical and/or thermal solid-liquidseparation. This can be solely a matter of a conventional precipitation.

However, it is more propitious to heat the dicarboxylic acid dialkylester or a dicarboxylic acid dialkyl ester mixture further up to abovethe boiling point and to evaporate it such that the polyester can beobtained by drying.

Naturally, a combination of precipitation and evaporation can also beused. Moreover, other solid, i.e. not soluble in dicarboxylic aciddialkyl ester, elements and compounds can be separated by means ofprecipitation.

Such separation can be achieved for example, in an adsorptive manner byfiltering or by separation in the centrifugal field.

By means of the method according to the invention it is moreoverpossible to reclaim plastics other than polyesters by dissolving indicarboxylic acid dialkyl ester. These can be polyolefins which go intosolution and are separated in preliminary stages during which lowertemperatures for dissolving are maintained.

The separation of polyester can also be carried out selectivelyfollowing the separation of the other types of plastics or respectivelyalso of other substances soluble in dicarboxylic acid dialkyl ester.

In the method according to the invention it is also very advantageousthat the consumption of fresh dicarboxylic acid dialkyl ester is reducedby the recycling of the evaporated and then condensed dicarboxylic aciddialkyl ester.

Besides the DBE already mentioned, other dicarboxylic acid dialkylesters or dicarboxylic acid dialkyl ester mixtures can also be used,e.g. diethyl esters or dimethyl esters of oxalic acid, malonic acid,succinic acid, glutaric acid and/or adipic acid.

Suitable polyesters for reclaiming are polyethylene terephthalate (PET)and/or polybutylene terephthalate (PBT).

If the reclaimed polyesters are to be used again, for example in thefood industry for packaging purposes or as drinks receptacles, thesolid-liquid separation should be carried out at least for so long thatthe proportion of dicarboxylic acid dialkyl ester is kept below 15 ppm.

If pure dicarboxylic acid dialkyl ester (DBE) is used to dissolve theplastics, a proportion of up to 20% polyester can be contained in thesolution. For certain applications, however, solutions of dicarboxylicacid dialkyl ester or corresponding emulsions in which dicarboxylic aciddialkyl ester is contained can be used advantageously.

With the invention, the costs for the polyester recyclate obtained canbe reduced by up to 50% in comparison with the corresponding newproduct.

The polyester portions of waste in the form of composite materials suchas metallized polyethylene terephthalate foils as well as wastecontaminated in some other way, covered with adhesive or printed, canalso be processed according to the invention and the metals be reclaimedin addition to the metals polyester.

In the method according to the invention it is moreover advantageousthat additional precipitation agents can be dispensed with and no secondliquid or even additional gas phase arises and consequently no specialprecipitation apparatus and no energy- and cost-intensive reprocessingof solvent and precipitation agent is necessary.

In tests, PET waste (shredded soft drink bottles) as well as flexibleprinted circuit boards and copper-coated PET foil were used at differentS/L ratios and kept at different temperature/time loads. No significantincrease in oligomers was noted by means of HPLC/MS measurements. Withthe aid of high-temperature headspace gas chromatography (HS-GC), a risein the proportion of acetaldehyde was noted, but otherwise no change andno transesterification, which has a negative effect on the properties ofpolyesters was noted.

The PET recyclate obtained had a slightly reduced glass-transitiontemperature TG of 80° C. by comparison with original PET.

Mechanical properties of PET recyclate are noted in the following table:

Tear strength Breaking elongation (N/mm²) (%) Sample Average valueAverage value DBE recyclate 8.0 12.5 PET flakes 58.6 4.6 amorph* PETflakes 18.0 1.0 crystalline *amorph = semicrystalline (transparent) PET

The method according to the invention will be explained in greaterdetail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures show:

FIG. 1 an example of process management for reclaiming PET waste, and

FIG. 2 a second process sequence for reclaiming polyesters from PETwaste.

In the process sequence schematically represented in FIG. 1 for anexample of a procedure according to the invention, the occurring andcollected PET waste, is comminuted and pre-cleaned in a first step.During the pre-cleaning, removal of detached metal parts, sieving,washing and also heavy-liquid separation can be carried out.

The shredded waste thus prepared is dissolved in a preliminary stage indicarboxylic acid dialkyl ester (DBE) at relatively low temperatureswhich are insufficient to dissolve polyester. In the next step, thesecomponents, for example other polymers, additives or other interferingsubstances are separated by solid-liquid separation, and possiblyremoved dicarboxylic acid dialkyl ester can be replaced again. Afterfurther heating, PET can dissolve in dicarboxylic acid dialkyl ester.This solution, which is rich in solids, is then subjected toprecipitation by lowering the temperature <150° C., excess dicarboxylicacid dialkyl ester being removed in this process and returned to thepreliminary or secondary stage described above or in place of thedicarboxylic acid dialkyl ester removed in the preliminary stage.

After the precipitation, the polyester is dried, i.e. it is heatedfurther possibly at a reduced pressure until the boiling point ofdicarboxylic acid dialkyl ester is exceeded. The gaseous dicarboxylicacid dialkyl ester can also be recycled after condensation.

The various method steps can be carried out in different reactionvessels which can be particularly advantageously designed for therespective application.

The dried PET can be subsequently extruded and further processed as PETrecyclate.

The example according to FIG. 2 differs from the example according toFIG. 1 in that the PET solution is cleaned by filtration, during whichinert dirt, cardboard, paper residues and metals are removed, beforeprecipitation and evaporation (e.g. spray drying). As a supplement tothe drying step conditioning also takes place before the dried PET ofimproved quality can be used again.

In this example, too, the excess DBE occurring in various method stepsis recycled.

1. Method for reclaiming polyesters or polyester mixtures from polyester waste or waste containing polyester, the steps comprising: heating at least one of polyester and waste containing polyester in a bath of solvent of dicarboxylic acid dialkyl ester to temperatures in excess of 150° C., dissolving the polyester in the dicarboxylic acid diallyl ester selected from the dialkyl esters of oxalic acid, malonic acid, succinic acid, glutaric acid and adipic acid, and extracting the polyester from the solution by means of solid-liquid separation.
 2. Method according to claim 1, characterised in that the separation of polyester and solvent is achieved by at least one of precipitation and evaporation of dicarboxylic acid dialkyl ester.
 3. Method according to claim 1, characterised in that the separation is carried out at a lowered temperature.
 4. Method according to claim 1, characterised in that dicarboxylic acid dialkyl ester containing polyester in dissolved form evaporates at temperatures in excess of 205° C. and the condensed gas phase is returned to a bath.
 5. Method according to claim 1, characterised in that impurities are separated from the solution by means of solid-liquid separation.
 6. Method according to claim 5 characterised in that the solution is filtered or separated in the centrifugal field.
 7. Method according to claim 1, characterised in that additional constituents of the waste which are soluble in dicarboxylic acid dialkyl ester are extracted separately in preliminary stages.
 8. Method according to claim 7, further comprising the step of separating polyolefins or polystyrene dissolved in dicarboxylic acid dialkyl ester in a preliminary stage.
 9. Method according to claim 1, characterised in that the waste is comminuted before being made into a solution.
 10. Method according to claim 1, characterised in that waste containing at least one of polyethylene terephthalate (PET) and polybutylene terephthalate (PMT) is used.
 11. Method according to claim 1, characterised in that the solid-liquid separation is carried out so far that a proportion of dicarboxylic acid dialkyl ester <15 ppm is kept in the polyester obtained.
 12. Method according to claim 1, characterised in that the dissolving of polyester in dicarboxylic acid dialkyl ester and the solid-liquid separation are carried out in separate reactors. 